A resonant optical scanner torsionally oscillates a mirror to repeatedly deflect a beam of light across an optical field. The light beam may originate from, e.g., a laser or a localized area of a source document being scanned. The scanner typically comprises a torsionally resonant rotor and a mirror attached to the rotor for oscillation therewith. Flexures, typically springy flat metal strips, attach the rotor to a frame. The rotor and flexures define a single-degree-of-freedom, resonant mechanical oscillator that is driven at its resonant frequency by a limited-rotation motor. The frame is attached to an optical bed, which typically also supports the optical source and target.
Resonant scanners have a relatively long life and can move a mirror as large as 1.5 in. in diameter at a frequency on the order of 200 Hz. Only more expensive rotating polygon scanners can match these mechanical capabilities. A resonant scanner advantageously uses a single reflective surface and, therefore, avoids the beam jitter associated with facet alignment errors in rotating polygon scanners. A resonant scanner can, therefore, provide accurate overlapping successive scans. However, and quite problematically, vibrational forces induced by the oscillations of the rotor limit the accuracy of the successive scans. Two types of vibrational forces can deform the optical bed and introduce optical errors.
Reaction torque from the torsional oscillations of the rotor create the first set of vibrational forces. The flexures must be sufficiently stiff to achieve a desired oscillation frequency, however such stiff flexures transmit the reaction torque to the frame. In prior art resonant scanners, the frame is rigidly mounted to the optical bed and all the reaction torque is transmitted to the optical bed. A typical rotor having an inertia of 10 g-cm.sup.2 and oscillating at 200 Hz with an amplitude of 15 degrees requires an excitation torque of about 3.5 in-lbf. This excitation is not severe because most optical beds are extremely rigid with respect to the local application of a torque having a vector perpendicular to the bed.
The rotor translates transversely each time the flexures deflect and these translations create the second, and more problematic, set of vibrational forces. As the rotor torsionally translates from its rest position toward one of its extreme positions, the flexures deflect and the straight-line distance between their respective ends decreases, i.e., the flexures become effectively shorter. As the rotor torsionally returns to its rest position, the flexures return to their undeflected state and effectively becomes longer again. As the rotor torsionally translates from its rest position toward its other extreme position, the flexures again deflect and become effectively shorter. Thus, the effective length of the flexures changes at twice the frequency that the rotor torsionally oscillates. These oscillatory changes in the effective length of the flexures cause the rotor to oscillate transversely with respect to the frame. In a typical resonant scanner with 0.84 in. long flexures, the rotor translates 0.0067 in. transversely when it deflects 15 degrees from its rest position. A typical rotor oscillating at 200 Hz. produces a transverse oscillation force of about 6.6 lb. at 400 Hz. This force can dynamically deform the optical bed and often produces an objectionable audible noise.
Prior art designers have attempted to reduce the transmission of the oscillatory forces by inserting vibration isolation pads between the frame and the optical bed. These pads are unsatisfactory because the forces involved have different lines of action and produce an oscillatory torque. A pad structure that reduces all these forces is mechanically complex, expensive, and limited in effectiveness.
It is, therefore, an objective of the present invention to provide a resonant optical scanner that imparts very low levels of vibrational forces to an optical bed.
Other objectives will, in part, be obvious and will, in part, appear hereinafter. The invention accordingly comprises an article of manufacture possessing the features and properties exemplified in the constructions described herein and the several steps and the relation of one or more of such steps with respect to the others and the apparatus embodying the features of construction, combination of elements and the arrangement of parts which are adapted to effect such steps, all as exemplified in the following detailed description, and the scope of the invention will be indicated in the claims.